Shockwaves are encountered when an aircraft reaches supersonic airspeeds. Such shockwaves exert significant forces on the thin layer of air around the aircraft, a component referred to as the boundary layer. These shockwaves interact with the boundary layer and, during strong interactions, can cause the boundary layer to deform. Bubbles and other boundary layer deformations increase drag and may also induce high levels of flow separation. These undesired boundary layer interactions accordingly bring about safety, performance, and longevity concerns, especially when the interactions occur inside of engine inlets.
Previous systems to alleviate such interactions have been developed. These systems bleed or circulate the air near the boundary layer to suppress shockwave induced flow separation. Active systems require some sort of ducting and/or pumping to bleed the air. Passive systems circulate airflow with holes above a cavity and require no automized labor.
Currently, most high-speed (above Mach 2) military aircraft employ active bleed transpiration systems for their engine inlets. Typical active systems include a plenum, covered with a fixed, flat porous or perforated plate. The plate draws the boundary layer air into the plenum and then through ducts. The ducts lead to a chamber which has a door. The door is controlled by a conventional actuator to open in predetermined increments to satisfy the varying bleed requirement for the shock compression. Expense, weight, drag, and complexity are the primary drawbacks to such conventional systems. In addition, much design effort is required to determine the location for the bleed intakes. This involves determining likely locations for shockwaves experienced during supersonic flight. Active bleed systems also lack an important benefit of passive systems that include injection upstream of the shock. The upstream injection allows additional thickening of the boundary layer upstream of shock, producing a system of weaker shocks, which thereby reduce wave drag and the intensity of the shock footprint.
Passive transpiration, which typically combines bleed downstream of the shock with flow injection upstream of the shock, is ideally preferable because it reduces the wave drag and intensity of the shock, and it does not require pumping power or ducting to or from the transpiration cavity. Current passive transpiration systems generally consist of a porous surface and a cavity underneath. The porous surface can be made of holes or slots. During supersonic flight, the changes in pressure will cause air downstream of the shock impingement to flow into the holes, through the cavity and then out through the holes upstream of the impingement. These systems have reduced mechanical complexity and expense compared to the conventional active transpiration systems. However, present models for passive transpiration systems have disadvantages. Transpiration rates are typically insufficient for effective boundary layer control due to the small hole size. Also, in general, the system requires holes or slots that are normal to the transpiration plate, creating a geometry that is significantly less effective than angled holes for bleeding purposes. Further, the holes can yield increased drag at lower Mach speeds or subsonic air flight because of their continuous open state. This potential leads to the same design concerns experienced in needing to determine the location of shock boundary interaction in a particular aircraft so the holes can be limited to that area. Otherwise, drag losses become too significant.
A more recently developed passive transpiration system limits drag by using holes that are micrometric in diameter on a sheet with a thickness on the order of micrometers. Subsonic drag effects are controlled, but other problems potentially arise. The thin nature of the sheet is a structural limitation which limits the porosity and effectiveness of the system. From a manufacturing perspective, difficulties are introduced as a result of the micrometric size of the holes. Because of the small size of the holes, they need to be made utilizing an electron beam technique or other similarly sophisticated micro machining technique, which creates a substantial expense as well as an impediment to mass manufacture.
Thus, there is a need for an improved passive transpiration method and system which addresses drawbacks in conventional systems. More specifically, there is a need for an improved passive transpiration system that effectively controls shock/boundary layer interaction at supersonic airflows and reduces drag at subsonic airflows. There is a further need for an improved system which provides for some flexibility in placement on an aircraft and uses relatively straightforward manufacturing techniques.